1. Field of The Invention
This invention relates to prestressed structural panels. More particularly, the present invention relates to a substantially planar structure having a face sheet in tension in at least one direction and a plurality of braces for maintaining the tension in the face sheet for use in a concrete form panel, among others.
2. Related Art
It is known to provide structural panels having a relatively thin face sheet that is bolstered or reinforced by one or more reinforcing members fixed to the rear side of the face sheet by means such as welding. Such panels are used in many applications, such as building walls, roofs, portable bridges, ships' hulls and so forth. In addition, a significant use for such panels is found in concrete forming systems, particularly in residential applications.
An application in concrete forming systems is found in U.S. Pat. No. 4,744,541, issued to Carlson et al. on May 17, 1988. Carlson et al. discloses a concrete form having a face sheet reinforced by a plurality of different types of reinforcement members. The face sheet has a face side that is presented to the poured concrete in the completed form, and a rear side to which the reinforcing members are fixed or attached by means such as welding. Such forms have established their value in the marketplace where they save significant amounts of labor compared with older competing forming systems, notably custom made wood forms. In addition, such forms typically lead to a superior finished concrete structure.
Such forms, however, have at least two related shortcomings. First, they are quite heavy, with a typical 3'.times.8' (0.9 m.times.2.5 m) panel weighing about 90 lbs. (41 kg.). Usually a single worker manipulates these form panels and moves them around on the job site. A lighter weight form panel could be expected to lead to increase worker productivity and fewer claims for job related injuries. Second, the pressure developed on the form panels from the hydrostatic head of the substantially fluid poured concrete and the expansion of the concrete upon setting distort the forms, causing the face sheet to bulge outwardly from the concrete in between the reinforcing members. One of the manifestations of this deformation is known as pillowing and results in a series of bulges which become increasingly pronounced from the top to the bottom of the form panel. The effect is frequently visible in the finished concrete product and detracts from the aesthetics of a formed concrete wall. Naturally, when the wall will be covered with an outer facade, such as stucco, or brick, the pillowing effect is not as important. Increasingly, however, architects are designing buildings with exposed concrete facades, sometimes molded to resemble brick or cut stone. In these applications the pillowing effect is substantially unacceptable.
Typically, solutions to these two problems are antithetical to one another. Decreasing the weight of the form reduces material costs, shipping costs, and may increase labor productivity, but leads to increased pillowing and decreased form life. In contrast, the pillowing effect can be virtually eliminated, but at the expense of considerably increasing the weight of the form panel. The increased weight of such a heavily reinforced form panel increases the cost of materials and shipping to prohibitive levels, as well as making it difficult at best for a single worker to handle the panel, leading to declines in productivity.
Accordingly, there is a need for a structural panel, such as a concrete form panel, that is both lightweight and strong enough to withstand the hydrostatic head pressure and the pressure caused by expansion of the setting concrete, of poured concrete, or other forces, and reduce pillowing between the reinforcing members.